316 EVENINGS AT THE MICROSCOPE 



Dr. Carpenter has described this texture so well that I 

 shall not apologize for quoting his words to you, especially 

 as you will have an opportunity here of testing their cor- 

 rectness, by personal observation. "It is," he remarks, 

 "in the structure of that calcareous skeleton, which prob- 

 ably exists, under some form or other, in every member of 

 this class, that the microscopist finds most to interest him. 

 This attains its highest development in the Echinida, in 

 which it forms a box-like shell, or 'test,' composed of 

 ^numerous polygonal plates jointed to each other with great 

 exactness, and beset on its external surface with 'spines,' 

 which may have the form of prickles of no great length, or 

 may be stout, club-shaped bodies, or, again, may be very 

 long and slender rods. The intimate structure of the shell 

 is everywhere the same: for it is composed of a network, 

 which consists of carbonate of lime, with a very small 

 quantity of animal matter as a basis, and which extends 

 in every direction (i.e. in thickness, as well as in length 

 and breadth), its areolse or interspaces freely communi- 

 cating with each other. These 'areolae,' and the solid 

 structure which surrounds them, may bear an extremely 

 variable proportion one to the other; so that, in two 

 masses of equal size, the one or the other may greatly 

 predominate; and the texture may have either a remark- 

 able lightness and porosity, if the network be a very open 

 one, or may possess a considerable degree of compactness 

 if the solid portion be strengthened. Grenerally speaking, 

 the different layers of this network, which are connected 

 together by pillars that pass from one to the other in a 

 direction perpendicular to their plane, are so arranged that 

 the perforations in one shall correspond to the intermediate 

 solid structure in the next, and their transparency is such 



